This invention relates to regulated power supplies of the type which utilize saturable core transformers and, more particularly, to high frequency switching power supplies of the inverter type.
Modern electronic equipment such as data processing equipment consumes considerable electrical energy, which energy must be provided at one or more relatively low voltage levels. Further, the power supplies providing this energy must be well-regulated in order to avoid the introduction of data errors into data processing equipment due to abrupt changes in the level of the supply voltage, and to transients.
Traditionally, a conventional power supply capable of providing well-regulated, low voltage, high current energy to a data processing unit has generally been comprised of an input step-down transformer which is coupled to a 50/60 cycle source; a high current, low voltage rectifier and filter; and a pass regulator. Differential comparison between the generated output voltage and a reference voltage has been the basis for a closed loop drive of the pass regulator. This classical approach offers excellent regulation, ripple, and dynamic response characteristics.
However, because of the necessity for utilizing a massive input transformer and for effecting direct regulation of high current, the traditional power supply of this type is very large, heavy and inefficient and also generates considerable heat which not only represents an energy loss in and of itself, but which also requires special cooling provisions. For these reasons, attention has therefore been directed to high frequency switching power supplies which provide a substantial reduction of the bulk and heat loss which characterize the prior art power supplies.
In a high frequency switching power supply, electrical energy from a line source is rectified and filtered directly to obtain a d-c unregulated voltage of, for example, approximately 150 volts. The unregulated d-c voltage is then applied directly to the primary winding center-tap of a high frequency inverter transformer. Switching transistors coupled to the ends of the primary winding are driven so as to alternately conduct at a relatively high frequency, e.g., 20 KHz, to introduce a high current at a low voltage into the inverter transformer secondary winding. The secondary current is rectified and filtered to provide the desired power for the data processing circuits. Regulation is typically achieved by monitoring the output voltage and appropriately adjusting the duty cycles of the switching transistors. Two classical power supply problems are thus eliminated. One, rather than a 50/60 KHz step-down transformer, a 20 KHz transformer is utilized. Thus, a transformer with a much smaller core can now be used and, second, regulation is effected on the high voltage, low current side of the system rather than the low voltage, high current side, such that the regulating system need not directly handle high current. As a result, heat losses are radically reduced.
The use of high frequency techniques requires care in the design and the selection of the components used, particularly the inverter transformer and the switching transistors. One particular problem related to the behavior of transformers can cause severe overheating and even catastrophic failure in the system. This problem is caused by a modest imbalance of the volt-seconds applied to the two halves of the inverter transformer primary winding. Whether this imbalance results from very slight differences in the pulse width of the drive to the power transistors, or from slight differences in the primary winding halves, or from other slight differences in the two primary drive systems, the results are the same namely a translation along the B-H curve of the transformer core until one side is driven into saturation. Core saturation causes the corresponding power switching transistors to draw excessive current which increases collector dissipation to destructive levels.
One prior art approach to solving this problem is disclosed in U.S. Pat. No. 3,873,903 by Philip W. Koetsch et al., which patent is assigned to the assignee of the present invention. In the power supply system disclosed therein, any difference in the current drawn through the power switching transistors is detected and this information is used to appropriately alter the duty cycle of one of the power switching transistor banks. This particular prior art approach can be classified as an electronic closed loop.
Another prior art circuit of interest is disclosed in U.S. patent application Ser. No. 632,129, entitled "Magnetic Circuit For A High Frequency Switching Power Supply", filed Nov. 14, 1975, by J. C. Jensen, which application is assigned to the assignee of the present application. The circuit of the referenced application utilizes a transformer employing two cores which are magnetically disposed in parallel. Individual reset windings for each core are provided, and the two reset windings are wound in opposite directions. Each reset winding is d-c biased whereby one core is correspondingly, magnetically biased in the direction of in-phase saturation in the positive direction and the other core is magnetically biased in the in-phase direction of saturation in the negative direction. Thus, during any half-cycle, the one core biased in the direction in-phase with the drive voltage will actuate. If saturation occurs, the other core then stands off the voltage, and the saturated core starts the succeeding half-cycle as far as possible from saturation in the opposite direction.
Although each of the aforementioned circuits appear somewhat satisfactory to accomplish a correction in the problem area, the present invention appears to be a viable alternate choice.